1. Field of the Invention
The present invention relates to a fixed focus lens suitable having a vibration control function and for 35 mm cameras, video cameras, electronic still cameras, and the like.
2. Description of the Related Art
Single lens reflex cameras have a mechanism to make a captured image and the viewfinder image coincide. The mechanism uses a mirror disposed just in front of the film to reflect light transmitted by the imaging lens and thereby guides the light to the optical viewfinder. Consequently, fixed focus lenses used in single lens reflex cameras require a long back focus, limiting the degree of freedom in terms of design. On the other hand, digital cameras can realize an equivalent operation by merely displaying on an electronic display, the image captured by the image sensor. Consequently, “mirrorless single-lens cameras” have appeared that realize a smaller apparatus size since the optical view finder and mirror for guiding the captured image thereto are omitted. Mirrorless single-lens cameras further offer the advantage of increased degree of freedom in terms of fixed focus lens design since the back focus of the imaging lens can be reduced. Accordingly, a great number of fixed focus lenses applicable to mirrorless single-lens cameras are available (see, for example, Japanese Patent Application Laid-Open Publication Nos. H9-325269, 2003-43348, and 2010-72276).
The optical system disclose in Japanese Patent Application Laid-Open Publication No. H9-325269 includes an internal focusing group and vibration control group. To facilitate powered movement of the internal focusing group and the vibration control group, preferably, the internal focusing group and the vibration control group should be configured by members of a relatively small diameter compared to other optical system members. However, since the focusing group is configured by 3 or so lenses and the vibration control group is configured by 2 or so lenses, sufficient reduction of the weight of the groups is prohibited, arising in the problems of increased load on the lens driving mechanism and increased power consumption.
Similar to the optical system disclosed in Japanese Patent Application Laid-Open Publication No. H9-325269, the optical system disclosed in Japanese Patent Application Laid-Open Publication No. 2003-43348 also includes a focusing group and a vibration control group. The focusing group in the optical system disclosed in Japanese Patent Application Laid-Open Publication No. 2003-43348 is configured by 2 or so lenses and thus, has a simpler, more desirable configuration than the optical system disclosed in Japanese Patent Application Laid-Open Publication No. H9-325269. Nonetheless, since the vibration control group is disposed near the image plane, movement of the vibration control group in a direction orthogonal to the optical axis of the vibration control group can easily become significant. To prevent this problem, the back focus has to be increased, which causes the overall length of the optical system to increase and thus, is undesirable.
The optical system disclosed in Japanese Patent Application Laid-Open Publication No. 2010-72276 includes within the optical system, a focusing group and a vibration control group, where a portion of the focusing group further functions as the vibration control group. This optical system has a desirable configuration that enables reduction of the overall length of the optical system. However, during focusing, since multiple lenses, the actuator of the vibration control group, and mechanical components are collectively moved, accurate termination of the movement is difficult to maintain. Furthermore, a large actuator for the vibration control group is necessary, arising in the problems of increased power consumption and increased lens barrel size.